Noncatalytic [2+2] Cycloaddition Providing Donor-Acceptor Type 2-Azetines Featuring the 2-Aminophenyl Groups

In contrast to the previously reported TiCl₄-catalyzed [2+2] synthesis of 2-azetines using internal alkynes and ethyl 3,3,3-trifluoro-2-((methylsulfonyl)imino)propanoate [CF₃C(═NMs)CO₂Et], electron-abundant terminal alkynes do not necessarily require a catalyst for the [2+2] cycloaddition process. In this study, we demonstrate that the 2-azetines, which were produced by using 2-ethynyl-N,N-dialkylanilines (ArC≡CH, Ar = 2-R₂NC₆H₄, R = alkyl), showed blue fluorescence (Φ = 4∼9%). The structural elucidation, using X-ray crystallographic analysis and density functional theory (DFT) calculations, of the fluorescent 2-azetines involved the pyramidalized flanking amino groups, as steric congestion prevented conventional π-conjugation between the amino and phenylene units. It is noteworthy that the C═C unit in the 2-azetine cycle plays a crucial role in the fluorescent character, and saturation of the double bond afforded the nonfluorescent azetidine derivative. Thus, the combination of the electron-donating pyramidal amino group and the electron-accepting π-molecular skeleton, including the appropriate bridging aryl spacer, is a promising molecular design for developing unique photo-functional N-heterocyclic materials. In addition, to investigate the o-aminophenyl effect on the 2-azetine framework, we employed the azole-substituted phenylacetylenes for the [2+2] cycloaddition. 2-(N-Pyrrolyl) and 2-(N-indolyl)phenylacetylenes facilitated the [2+2] cycloaddition process predominantly and afforded the corresponding 2-azetines exclusively, whereas the [2+2] synthesis reported so far included the byproducts.